The aforementioned applications have in common that they require a large amount of air to be filtered with high particle filtration efficiency. While a single filter cassette has the capacity for filtering more than 1,000 m3 per hour, with a typical filter size of 592 mm×592 mm×300 mm or 610 mm×610 mm×300 mm filtering about 2,500 to 5,000 m3 per hour, a great number of filter cassettes are used in parallel in order to filter an amount of air of more than 10,000 m3 per hour or even more than 50,000 m3 per hour, and sometimes even much more than that. The filter cassette or cassettes are mounted in a partition separating an upstream volume generally referred to as the “dirty air section” and a downstream volume generally referred to as the “clean air section”. The partition may be in the form of a wall with openings in which the filter cassettes are mounted or may be in the form of a rack defining a plurality of openings in which the filter cassettes are mounted so as to create a substantially airtight partition between the dirty air and clean air sections. In certain applications the great number of filter cassettes are provided in a filter house sufficiently large for operating staff to walk through and remove and replace individual filter cassettes when they are clogged or defective.
U.S. Pat. No. 6,368,386 relates to an air filter system in an air intake stream of a gas turbine. Particulate material is filtered from the intake air at a first stage air cleaner and directed to a second stage air cleaner where moisture and, particularly, salt are removed from the intake air. The second stage air cleaner typically includes the aforementioned filter cassettes. Filter material that can advantageously be used as a filter media in the second stage air cleaner is described e.g. in EP 1 674 144 A1. The structure of common filter cassettes is described e.g. in WO 2007/103408, EP 0 560 012 B1 and EP 0 723 800 B1. Accordingly, the filter cassettes typically comprise a plurality of filter panels arranged so that pairs of panel filters form V-pockets extending from the filter cassette's upstream end to the filter cassette's downstream end. Each filter panel is composed of multiple pleats of filter media extending generally parallel to the overall filtration path, so that air or gas to be filtered passes through the pleats in a generally straight manner. The filter panels are mounted in a casing and are air tightly fitted in a mounting frame at the filter cassette's upstream end, or in a few applications at the filter cassette's downstream end. The mounting frame provides a mounting face for mounting the filter cassette to a corresponding mounting face of the partition so that the filter cassette extends into and through the opening of the partition into the clean air section. Staff can then easily remove and replace the filter cassette from the dirty air section side.
It is the object of the present invention to improve the known filter cassettes and filter arrangement, in particular in respect of filtering efficiency, lifetime, and pressure drop.